Endocrine Abstracts

Case History: A 49 year old man was referred from primary care with clinical features of thyrotoxicosis, including weight loss, anxiety and palpitations, over the previous four months. The patient had a 20 year history of abnormal thyroid function, characterised by elevated fT3 and fT4 with normal TSH. He had been previously assessed by the regional thyroid service 12 years prior and diagnosed with thyroid hormone resistance syndrome (RTH). This was based on clinical and biochemical assessment including TRH stimulation test, demonstrating an exaggerated response (TSH rising from 1.36 mIU/l to 14.1 mIU/l at 60 minutes) and a non-elevated alfa-subunit. Previous pituitary imaging was reported as normal. There was no family history of RTH and genetic tests failed to identify a mutation of the thyroid hormone receptor beta-gene.

Investigation & Results: On review in clinic, he appeared euthyroid, with no goitre present. Carbimazole (30 mg) had been commenced in primary care. TFTs showed: TSH 7.06 mIU/l, fT4 17.9 pmol/l. Carbimazole was then stopped and TFTs repeated: TSH 5.21 mIU/l, fT4 27.3 pmol/l, fT3 8.5 pmol/l. Pituitary MRI showed a normal sized pituitary with a possible sub-centimetre left sided pituitary lesion.

TSHoma was suspected and the patient was given a 3 month trial of somatostatin analogue (SA).

There was improvement, but no normalisation of TFTs. SA was initially stopped, but then recommenced, demonstrating biochemical normalisation and symptomatic improvement.Methionine PET-CT was performed (off SA) which demonstrated a skew of tracer towards the left inferior paramedian aspect of the gland. Repeat Methionine PET-CT (on SA) showed a loss of this asymmetry, in-keeping with a small thyrotroph tumour.

Treatment: The patient proceeded to transsphenoidal surgery for resection of the presumed TSHoma. Histology confirmed TSH immune-positive pituitary adenoma. SA was discontinued and post-operative TFTs have remained normal (TSH 1.61 mIU/l, FT4 15.7 pmol/l, FT3 4.3 pmol/l). The patient is well and is undergoing T3 suppression test to assess cure of the TSH-oma.

Conclusion and Points for Discussion: This case shows a patient, originally misdiagnosed with thyroid hormone resistance, but subsequently found to have TSH-oma 20 years later. It is unusual for a TSH-microadenoma to be resected after such a prolonged period. The case illustrates the effective use of both somatostatin analogues and Methionine PET in the diagnosis of TSHoma.